The present invention relates to fluid controllers of the type used to control the flow of fluid from the source of pressurized fluid to a fluid pressure operated device, such as a steering cylinder.
A typical fluid controller of the type to which the present invention relates includes a housing which defines various fluid ports, and further includes a fluid meter and valving, and an arrangement for imparting follow-up movement to the valving in response to flow through the fluid meter. The flow through the controller valving is directly proportional to the area of the main variable flow control orifice, which, in turn, is proportional to the rate at which the steering wheel is rotated. Furthermore, the area of the main variable flow control orifice has a known relationship to the displacement of the controller valving.
Fluid controllers of the type to which the invention relates are frequently used on large, heavy vehicles, such as loaders, backhoes, and various other construction vehicles. Steering such a vehicle requires a relatively large flow of fluid through the controller to the steering cylinder per revolution of the steering wheel. However, because typical prior art controllers have been designed such that all flow to the cylinder must first pass through the fluid meter, controllers for large vehicles have required fluid meters with relatively large displacements. Such large displacement fluid meters add substantially to the size and cost of the controller.
Therefore, it has long been an object of those skilled in the art to provide a steering system, including a fluid controller, in which the total flow through the steering system is substantially greater than the flow through the controller, but with the overall system flow being related to the flow through the controller in a known manner. See for example U.S. Pat. No. 4,052,929 in which the controller receives fluid from one pump and generates a pilot signal to control a pilot operated valve which receives fluid from a second pump. The total steering flow comprises the flow through the pilot operated valve plus the flow from the controller. Such a system is theoretically satisfactory, but the cost of such a system becomes nearly prohibitive because of the addition of the pilot operated valve and the second pump.
More recently, there has been an attempt to provide a flow to the steering cylinder which is greater than the flow through the fluid meter by having the full amount of desired steering flow enter the controller, with one portion flowing through the controller valving and fluid meter in the normal manner, and the remainder of the fluid flowing through a pressure regulating device and a bypass throttle. These two portions of fluid recombine within the controller and flow to the steering cylinder. See U.S. Pat. No. 4,566,272. It is possible that the performance of a controller made in accordance with U.S. Pat. No. 4,566,272 would be satisfactory, however, the addition of a pressure regulating valve within the controller, and the associated structure would still add substantially to the cost of the controller, and in many applications would require substantial redesign of at least the controller housing in order to accommodate the addition of such a valve.
In vehicle applications in which provision has been made for a very large steering flow, an additional problem has arisen. Typically, the full system flow (i.e., the displacement of the fluid meter), is available upon initiation of steering action. The result is that there is sufficient steering flow at higher valving displacements to achieve a satisfactory number of steering wheel turns, lock-to-lock. However, at relatively smaller valve displacements, such as for small steering corrections, there is too much flow, and the steering appears too responsive to the vehicle operator.